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Vermiculite
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Vermiculite
Vermiculite is a hydrous phyllosilicate mineral which undergoes significant expansion when heated. Exfoliation occurs when the mineral is heated sufficiently; commercial furnaces can routinely produce this effect. Vermiculite forms by the weathering or hydrothermal alteration of biotite or phlogopite. Large commercial vermiculite mines exist in the United States, Russia, South Africa, China, and Brazil.
Vermiculite was first described in 1824 for an occurrence in Millbury, Massachusetts. Its name is from the Latin vermiculare, "to breed worms", for the manner in which it exfoliates when heated.
It typically occurs as an alteration product at the contact between felsic and mafic or ultramafic rocks such as pyroxenites and dunites. It also occurs in carbonatites and metamorphosed magnesium-rich limestone. Associated mineral phases include: corundum, apatite, serpentine, and talc. It occurs interlayered with chlorite, biotite and phlogopite.
Vermiculite is a 2:1 clay, meaning it has two tetrahedral sheets for every one octahedral sheet. It is a limited-expansion clay with a medium shrink–swell capacity. Vermiculite has a high cation-exchange capacity (CEC) at 100–150 meq/100 g. Vermiculite clays are weathered micas in which the potassium ions between the molecular sheets are replaced by magnesium and iron ions.
This process involves mixing exfoliated vermiculite with inorganic bonding agents such as sodium silicate, cement (specific quantities), and other compounds, such as those containing potassium,[clarification needed] to produce an 'earth damp' mixture. This material is then hydraulically pressed into shape in a mold and then heat cured at temperatures up to 180 °C for up to 24 hours, depending upon the thickness of the moulded part. Such parts can withstand service temperatures of up to 1150 °C and are often used in the aluminium smelting industry as back-up insulation behind the carbon cathode in the pot cells which contain the molten mixture of cryolite and alumina. The moulded shapes and boards are used in:
Exfoliated vermiculite is added to a calcium silicate slurry. This is then dewatered by pressing or by using one of the Fourdriner/Magnani/Hatschek processes to form a flat board which is then heat cured under pressure (typically 10–15 bar) for periods of up to 24 hours.
Finer grades of exfoliated vermiculite are being used in brake linings primarily for the automotive market. The properties of vermiculite that make it an appropriate choice for use in brake linings include its thermal resistance, ease of addition to other raw materials to achieve a homogeneous mix, and its shape and surface characteristics.[citation needed]
Exfoliated vermiculite (typically the finer grades) can be added at site to Portland cement and other aggregates, rheological aids, and water to produce roof and floor concrete screeds (i.e. top layer screed coats) which are lightweight and insulating. In many cases, vermiculite-based roof screeds are used in conjunction with other insulation materials, such as polystyrene board, to form a total roofing system. A bituminous binder can also be used with exfoliated vermiculite to produce a dry, lightweight roof screed which has the advantages of low thermal conductivity, low moisture content, and ease of placement (by pouring from the bag and then tamping).[citation needed]
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Vermiculite
Vermiculite is a hydrous phyllosilicate mineral which undergoes significant expansion when heated. Exfoliation occurs when the mineral is heated sufficiently; commercial furnaces can routinely produce this effect. Vermiculite forms by the weathering or hydrothermal alteration of biotite or phlogopite. Large commercial vermiculite mines exist in the United States, Russia, South Africa, China, and Brazil.
Vermiculite was first described in 1824 for an occurrence in Millbury, Massachusetts. Its name is from the Latin vermiculare, "to breed worms", for the manner in which it exfoliates when heated.
It typically occurs as an alteration product at the contact between felsic and mafic or ultramafic rocks such as pyroxenites and dunites. It also occurs in carbonatites and metamorphosed magnesium-rich limestone. Associated mineral phases include: corundum, apatite, serpentine, and talc. It occurs interlayered with chlorite, biotite and phlogopite.
Vermiculite is a 2:1 clay, meaning it has two tetrahedral sheets for every one octahedral sheet. It is a limited-expansion clay with a medium shrink–swell capacity. Vermiculite has a high cation-exchange capacity (CEC) at 100–150 meq/100 g. Vermiculite clays are weathered micas in which the potassium ions between the molecular sheets are replaced by magnesium and iron ions.
This process involves mixing exfoliated vermiculite with inorganic bonding agents such as sodium silicate, cement (specific quantities), and other compounds, such as those containing potassium,[clarification needed] to produce an 'earth damp' mixture. This material is then hydraulically pressed into shape in a mold and then heat cured at temperatures up to 180 °C for up to 24 hours, depending upon the thickness of the moulded part. Such parts can withstand service temperatures of up to 1150 °C and are often used in the aluminium smelting industry as back-up insulation behind the carbon cathode in the pot cells which contain the molten mixture of cryolite and alumina. The moulded shapes and boards are used in:
Exfoliated vermiculite is added to a calcium silicate slurry. This is then dewatered by pressing or by using one of the Fourdriner/Magnani/Hatschek processes to form a flat board which is then heat cured under pressure (typically 10–15 bar) for periods of up to 24 hours.
Finer grades of exfoliated vermiculite are being used in brake linings primarily for the automotive market. The properties of vermiculite that make it an appropriate choice for use in brake linings include its thermal resistance, ease of addition to other raw materials to achieve a homogeneous mix, and its shape and surface characteristics.[citation needed]
Exfoliated vermiculite (typically the finer grades) can be added at site to Portland cement and other aggregates, rheological aids, and water to produce roof and floor concrete screeds (i.e. top layer screed coats) which are lightweight and insulating. In many cases, vermiculite-based roof screeds are used in conjunction with other insulation materials, such as polystyrene board, to form a total roofing system. A bituminous binder can also be used with exfoliated vermiculite to produce a dry, lightweight roof screed which has the advantages of low thermal conductivity, low moisture content, and ease of placement (by pouring from the bag and then tamping).[citation needed]
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